An integrated circuit device typically includes output buffers for driving resistive-capacitive-inductive loads external to the integrated circuit device. Capacitive components of the external loads may include capacitances of bond wires and package pins in an integrated circuit package containing the integrated circuit device, as well as input capacitances of gates connected to the output buffers. Inductive components of the external loads may include inductances of power and ground lines external to the integrated circuit device that are electrically connected to the output buffers. For instance, the power and ground lines may be conductive traces on a printed circuit board containing the integrated circuit device. A resistive component of the external loads includes resistances of interconnects electrically connected to the output buffers and may also include pull-up or pull-down circuits electrically connected to the interconnects. For example, the interconnects may be conductive traces on a printed circuit board that are connected to package pins of the integrated circuit package.
A traditional output buffer of an integrated circuit device includes a chain of inverters in which the sizes of transistors are selected based on the external load of the output buffer. Because of the transistor sizing in the inverter chain, output buffers of an integrated circuit device may have high slew rates. In such an integrated circuit device these output buffers may switch simultaneously, causing an unacceptably high peak current in the integrated circuit device. Additionally, the simultaneous switching of the output buffers may cause power noise, resulting in an unacceptable voltage drop in power supplied to the output buffers. Moreover, simultaneous switching of the output buffers at high slew rates may generate an unacceptable level of electromagnetic interference emitted from the integrated circuit device.
In light of the above, a need exists for controlling the slew rate of an output buffer. A further need exists for increasing the power noise immunity of an output buffer.